This invention relates to a rear derailleur for a bicycle, and more particularly to a rear derailleur for a bicycle, which is adapted to switch a driving chain to a selected one of multi-stage sprockets for changing the bicycle speed.
This kind of derailleur has a pantograph mechanism comprising a base member supported to the bicycle frame, a pair of linkage members supported swingably to the base member, and a movable member supported to the linkage members, the movable member carrying a chain change-over cage having a guide pulley and a tension pulley.
The change-over cage of a conventional rear derailleur comprises one pulley plate supported to the movable member and one guard plate having substantially the same shape as the pulley plate, both the plates being opposite to each other and rotatably carrying the guide and tension pulleys between these plates, and the pulley plate is subjected to the force of a tension spring supported to the movable member.
When the rear derailleur is used for the bicycle, the base member is disposed axially outwardly of a rear hub carrying the multi-stage sprockets and is supported to the frame, and a driving chain stretched across a selected sprocket and crank means is, along its length, placed onto the guide and tension pulleys to travel in a Z-like manner from the tension pulley to the guide pulley, the chain being applied with a given tension by the tension spring.
A control wire is operated to transform the pantograph mechanism and move the movable member axially of the multi-stage sprockets, so that the driving chain is guided by the guide and tension pulleys and switched to a selected sprocket for changing the bicycle speed.
In the conventional derailleur constructed and used according to the foregoing, the chain guard comprises a guard portion for the chain placed on the guide pulley and that for the chain placed on the tension pulley, which guard portions are connected. In order to place the chain onto the tension and guide pulleys, the chain should be inserted through a space enclosed by the surrounding pulley, pulley plate and guard plate. Therefore, the endless chain is cut for insertion through the space.
Hence, it is very difficult to place the chain onto the pulleys which results in the problem of poor workability. The above problem can be solved by cutting the guard plate for inserting the chain therethrough. However, another problem will arise in that the chain, when in use, can disengage from the respective pulleys and escape through the cut portion.
This invention has been designed to overcome the aforesaid problems. A main object of the invention is to provide a rear derailleur for a bicycle, which is capable of simply placing the endless chain onto the pulleys and which prevents the chain from unexpectedly escaping from the pulleys. Another object of the invention is to provide a rear derailleur for a bicycle, which is simple in construction and inexpensive to manufacture because a particular member is not required for preventing escape of the chain.
This invention is characterized in that first and second guard plates are separately provided for the guide and tension pulleys, and between the first and second guard plates is formed a gap which accepts insertion of the chain therethrough, and the gap being closed by use of a tension spring.
In greater detail, a tension spring having first and second ends of connection is carried onto a pivot shaft through which the pulley plate is supported to the movable member. The first end of the tension spring is retained by the moveble member and the second end extends to a shaft of the tension pulley along the outer surfaces of first and second guard plates and is retained by the pulley shaft in the vicinity of the axial end thereof to thereby close the aforeseaid gap.
Consequently, the second end of the tension spring is released from the pulley shaft to open the gap, whereby the chain, being endless, can be placed onto the pulleys through the opening gap. After the chain is placed onto the pulleys, the second end of the tension spring is re-connected to the pulley shaft and closes the gap to thereby ensure that the chain is prevented from unexpectedly escaping from the respective pulleys.
These and other objects and novel features of the invention will be more apparent by the following description of an embodiment thereof in accordance with the accompanying drawings.